Sony Vegas - A few questions

[Simbo]

Hi,

I have been using Sony Vegas 9 Pro but there are a few things that im puzzled by. It would be great if someone could clear a few things up for me.

Firstly I was hoping someone could explain the 'override compression buffer' setting during rendering. What does this do? Does it need to be checked or unchecked?

Also there is another rendering setting that im not sure about. 'Seconds per keyframe'. Could this also be explained please?

Lastly my AVCHD clips are recorded in 1080i. I have the project settings matching this. I am rendering in WMV format (needs to be this for streaming purposes) but Vegas tells me it is rendering in progressive. Does anyone know if it is possible to change the settings to interlaced for this codec? Also would this have any adverse affect on the output quality if it is progressive instead of interlaced?

Thanks for your help.

[Arkady Bolotin]

Hi Simbo,

It is my understanding that all your questions are about the settings of Windows Media Video 11 format (WMV) rendering.

When rendering you project in this format, “Override default compression buffer” setting means that you can choose the compression rate other than the default value. However, since your clips are recorded in AVCHD format (with a variable bit rate), that makes this setting irrelevant, and you should deselect its check box.

“Seconds per keyframe” setting means the interval between video keyframes. In video sequence not all frame are equal: some of them – so called keyframes – contain data necessary to draw other frames. The faster the pace of motion in your film, the more frequently you should insert those keyframes to provide higher rendering quality. The usual range for this setting is from 20 seconds to 1 second, so the default 5 is close to the mean value.

“Is possible to change the settings to interlaced for this codec”? The answer is no, because the WMV codec does what it says, “Use this setting for high-quality HD video playback on computer”. Computer displays are all progressive scan type, so when viewing the video on a computer, interlacing is ignored.

Therefore, to make the rendering results more predictable, I recommend changing your project properties concerning interlacing: Field order to “None (progressive scan)” and Deinterlace method to “Blend fields”.

[Simbo]

Thats great...thanks for the clear explanation.

That leads me to one more question though.

Will the fact that the original clip is interlaced and the output file is progressive have an affect on quality? I know some quality is lost during compression but will that difference cause more?

Would I see better results using a codec with an interlaced setting?

[Arkady Bolotin]

“Will the fact that the original clip is interlaced and the output file is progressive have an effect on quality?” The answer is yes and no.

Theoretically speaking, the template, which specifies the parameters used for rendering your project, should match your project settings such as frame size, pixel aspect ratio, and frame rate and field order. Here is a tip for you: in the Vegas, those templates are displayed with an equal sign (=) in the corresponding list.

On the other hand, in your case (interlaced video material is supposed to be viewed on computer displays which support only progressive scanning) there are no matching templates. Therefore, as long as you are not going to display your video on CRT televisions, you must apply de-interlacing.

With regard to de-interlacing algorithms, they should try to avoid interlacing artefacts and not sacrifice image quality in the process. However, since this is a very complex task, there is no doubt that 'ideal' deinterlacer is practically impossible.

But the good news is the loss in image quality due to proper deinterlacing is practically so negligible that hardly anyone could tell the difference.

[Simbo]

Thank you. You really have explained things very well.

May I also ask whether I should use a constant bitrate (CBR) or variable (VBR)? Under the video tab there are a few different options.

Many thanks.

[Arkady Bolotin]

Roughly speaking, bitrate represents the amount of information, which is stored per unit of time of a recording: thus the higher bitrate, the higher the detail level of a recording.

For example, standard-definition television quality (with MPEG-2 compression applied) is about 3.5 Mbit/s; while DVD quality is just 5 Mbit/s. High definition is a different story: quality of the HDTV broadcasting is 8 to 15 Mbit/s, and – cream of the crop – Blu-ray Disc is 40 Mbit/s at its maximum.

When recording or playing back video material, constant bit rate (CBR) means that the rate at which a codec's output data should be consumed is constant. The only case when one can benefit from the CBR use is streaming multimedia thru limited capacity network. Unlike variable bitrate (VBR), CBR is not an optimal choice for encoding as it does not allocate enough data for detail-rich frames (resulting in degraded quality) while wasting data on simple frames.

Therefore, the advantages of VBR are that it produces a better quality-to-space ratio compared to a CBR file of the same data. The video is encoded more accurately with fewer bits used in less demanding frames and more bits used in difficult-to-encode frames.

The drawbacks of VBR are that it takes more time to encode video material, as the process is more complex.

So, my advice is this: if the time for a project completion isn’t an issue for you, use VBR exclusively.

[Simbo]

Thanks again.

When rendering in WMV format there are 3 different options for VBR. They are 'Bit rate VBR (peak), 'Quality VBR' and 'Bit rate VBR'.

Is there much difference between them?

[Arkady Bolotin]

Please look at the screenshots I’ve made to simplify your task:

Quality VBR audio setting is chosen to ensure high sound quality; the sample rate of 44.100 kHz is the best for network media.

Also, for video encoding Quality VBR setting is the simplest yet acceptable choice (other options as Bit rate at peak or Bit rate VBR are more complicated since you need to calculate these values before rendering).

[Simbo]

Thats excellent. You have answered all of my questions very well.....for now at least!!

Thank you again for taking the time to explain things for me.

[Simbo]

Hi again!

I was hoping someone might be able to answer another question I have.

Ive been playing around with the New Blue FX Cartoon plugin with the aim of starting a clip off looking 'cartoony' and then gradually fading it out so that the same clip ends in its original look.

The cartoon effect has a timeline so I have been adding points on the timeline and turning down the effect at each point to slowly fade it out. When I go to preview the clip though nothing happens!

Has anyone got any advice on how to get it to work?

I hope I've explained myself ok.